Monday, 28 August 2017

Mud, mud and more mud: salt marsh sampling in Essex... by Helen Brooks

Hi, I’m Helen and I’m a 1st year PhD student at the University of Cambridge, in
partnership with BUFI, the BGS University Funding Initiative. My research looks at sediment
properties of salt marshes and tidal flats and how these relate to the
stability of salt marshes. This means I get to spend a lot of time sampling in
the field, which is great fun (if the weather is good)!

Why are we interested in salt marsh
stability?

Salt marshes
help protect the land behind them from flooding and erosion, however marsh
areas are declining globally. My research will improve models that simulate
future marsh evolution, particularly in a changing climate. This will help us
to understand whether marshes will continue to protect us from flooding in the
future.

Sunset on Tillingham marsh in February

What exactly do I sample?

I’m
interested in the sediment properties and how these relate to erosion
resistance. During my PhD I hope to look mainly at the sediment shear strength,
compressibility, shrink-swell behaviour and plasticity (i.e. how the sediment
behaves at different moisture contents). Most of these tests will be done in
the lab and will require undisturbed samples. That means a sample where we try
to keep in the in situ condition. For
anyone who hasn’t done undisturbed sampling before, you’ll probably think I’m
crazy by the end of this paragraph. I’m going to try and explain the process of
collecting the samples. Essentially, you dig the outside of a pit, leaving the
central section untouched (see photo). Then you carefully remove the vegetation
and the uppermost sediment (and by 'carefully' I mean 'using small kitchen knives to
remove 2 cm2 chunks of sediment, bit by bit'!). The next step is to
place the sampling equipment on the surface, push down gently by 1-2 mm, then
scrape the excess 1-2 mm of sediment carefully from around the side of the
equipment (see photo). Then we repeat pushing down and scraping the excess
sediment until the sediment reaches the top of the sample! In total, digging
the pit and taking the sample can frequently take three hours! Finally, the
samples are wrapped in A LOT of bubble wrap, placed on trays and packed in
boxes to reduce disturbance during transport! But after all this, do you know
what the best part of the day was? Enjoying a glass of freshly squeezed orange
juice! To explain: the rest of my research group were in the field flying a
drone over the marsh and used halved oranges as ground control points (novel, I
know!). However, so as not to waste the oranges, these were squeezed before we
went out to the field!

Digging the outside of the pit, leaving the central section untouched.

Carefully pushing the sampler into the sediment, then scraping away the sediment around the outside. This particular sample is nearly done, as the sediment inside the sampler has now reached the top of the sampler.

Where do I sample?

Most
recently, I was sampling at Tillingham Marsh in Essex. We spent two days
meticulously taking undisturbed samples in the glorious sunshine (though
unfortunately it isn’t always like this!), before carefully transporting the samples
back to Cambridge. The samples were taken from the lower marsh, close to,
but not on, the tidal flat.

Tillingham Marsh in summer

What next?

I’m now
about to start work on these samples at BGS Keyworth. This will involve three
weeks of more sample preparation, plus lots of loading the samples into
machines which allow me to assess properties such as consolidation, shear
strength and shrink-swell behaviour.

If you’re
interested in finding out more about my project, please follow me on Twitter at
@hbrooks94